What are plasticizers?
Plasticizers are generally clear, organic, liquid materials that are added to PVC formulation to obtain a flexible film to enhance both processability and serviceability of the end product. Plasticizers are very compatible with PVC resin and become an integral permanent part of the film matrix. Generally Phthalate, Adipate and Polymeric Plasticizers are used in the formulation of PVC geomembrane and are referred to as primary plasticizers because of their excellent compatibility with the PVC resin even at very high levels.
What is the purpose of plasticizers?
PVC liner production involves mixing or blending PVC resin, stabilizers, plasticizers and pigments. The plasticizer softens the polymer to make it pliable to aid in Calendering (manufacturing) process and provide a flexible sheet which is easy to seam in both factory and field conditions by a variety of methods. When plasticizer is blended with the PVC, a portion of it forms an intimate bond with the PVC, while the remainder is captured in the polymer matrix.
How is plasticizer loss measured?
The rate of loss of plasticizer depends on plasticizer type, temperature, sheet thickness, environment and exposure time. The actual mechanism behind this loss is migration of plasticizer from the surface membrane. Generally higher molecular weight plasticizers used in a variety of product applications including PVC geomembranes and medical bags, have significantly improved plasticizer loss rates over lower molecular weight plasticizers used in low end products such as shower curtains, book binding, etc.
Plasticizer loss is evaluated as part of the quality control certification process using ASTM D1203 for volatility and ASTM D3083 for water extraction. The test procedure for ASTM D1203 exposes the samples to activated carbon at the elevated temperature of 70° C for 24 hours. Samples are then removed, reconditioned and reweighed. The NSF International standard 54 for PVC geomembrane requires that the volatile loss be between 0.5 percent to 1.5 percent depending upon the geomembrane thickness. This assures that any component in the PVC geomembrane, including plasticizer, is not volatile and is permanent.
The water extraction test ASTM D3083 immerses the geomembrane in water for 24 hours at 50+ 2° C. Low extraction losses of between 0.25 percent to 0.35 percent are specified according to the requirements of NSF standard 54. This verifies that the PVC geomembrane will have low plasticizer loss in an aqueous environment such as the typical leachate.
How is plasticizer loss recognized and what effect does it have on the physical properties of the film and its ability to perform?
Plasticizer loss can be recognized by the following factors:
Changes in the liner do not adversely affect the performance in most applications. Increases in modulus and tensile only show that the geomembrane is stronger. Reduction of elongation, although it does occur, is usually well above 100 percent and that is still well above most design criteria of five to 15 percent typically used in geomembrane construction.
- Reduction in total weight of the sheet
- Slight reduction in overall film thickness
- Increase in tensile strength
- Increase in modulus at 100 percent elongation
- Reduction in ultimate elongation percentage
What is the plasticizer loss rate of PVC geomembranes?
Approximately 30-35 percent of the PVC formulation is plasticizer. The actual loss rate is determined by the application in which it is used. The worse case is a situation where there is the largest gradient of organic components between the geomembrane and the surrounding environment. A canal liner in pure water would actually be a more severe application than a liner used as a municipal landfill liner. The reason for this is that the water is lower in organic components than a typical landfill leachate. Also, the water in the canal is constantly changing compared to the landfill that is stagnant and will eventually reach a steady state at the leachate/geomembrane interface. This is why a study conducted by the United States Bureau of Reclamation is useful in evaluating the plasticizer rate loss of PVC geomembrane.
In this study, the USBR evaluated a 10-mil PVC geomembrane which was used to line a canal system in the 1950's. the Bureau of Reclamation removed samples of 10 mil PVC from four different sites at internals of 2,4,5,9,14 and 19 years after installation to compare the physical properties of in service membranes to a control sample. Below is a summary of the physical properties of 10 mil PVC as tested by the USBR. See Figure1.
Service Years 2 4 5 9 14 19 Original Specification Value Plasticizer Content Remaining 92.1% 67.7% 67.8% 65.6% 58.0% 54.3% None Tensile Strength 30.7 (L) 28.8 (T) 29.3 (L) 31.8 (T) 29.6 (L) 28.7 (T) 34.6 (L) 28.7 (T) 26.0 (L) 33.3 (T) 28.6 (L) 26.9 (T) 17 lb. F/in Elongation 167 (L) 233 (T) 123 (L) 176 (T) 153 (L) 208 (T) 180 (L) 225 (T) 169 (L) 225 (T) 151 (L) 188 (T) 225% Tear Resistance 2020 (L) 3920 (T) 1190 (L) 990 (T) 1440 (L) 1760 (T) 1825 (L) 3490 (T) 1660 (L) 2210 (T) 450 (L) 1300 (T) 1500 grams
This study shows that after four to five years the plasticizer migration rate decreases drastically. Part of the reason is that as plasticizer is lost the PVC matrix binds to the remaining plasticizer even more strongly. Although this was only 10 mil film the Bureau of Reclamation reported that over half the plasticizer was still present after 19 years. The geomembrane still exceeded most of the original project specifications and is still a serviceable membrane today, almost 40 years later.
The following should be considered when reviewing plasticizer loss data:
- Many advances in PVC resin, plasticizer and PVC compounding technology have been made since the 1960's and the 1970's. These improvements have meant that the PVC geomembranes of today have even greater resistance to plasticizer migration. This work continues today.
- Certification tests to check the permanence of the plasticizer using volatility, ASTM D1203 and water extraction ASTM D3083 assure the geomembrane supplier that the product will meet design criteria for years in the future.
- Case histories such as the USBR study, EPA 9090 testing and laboratory extraction tests prove repeatability and consequently, the reliability of PVC geomembranes in a wide range of applications.
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